It is known to provide, in an interior of an image forming apparatus, a detector to detect a final image on a recording medium or sheet and many proposals have been made so far. The image formed on the recording medium is detected by the detector and the image quality is determined based on the detection result, or used as feedback to recursively change or correct image forming condition.
To detect the image formed on the recording medium, in the case of using a test pattern that is not an actual image, the number of sheets used must be minimized. Accordingly, a method of using both sides of the recording medium has been proposed.
JP-2005-202028-A discloses a technique to automatically detect both sides of the sheet in one test printing. Specifically, when a color detector to detect a patch on the recording medium is disposed at a duplex-print conveyance path and a discharge conveyance path and the patch is detected by the color detector, a first surface and a second or backside surface are detected in one test printing. This technique has an advantage that the test patches on the first and second surfaces of the recording medium can be detected at once. However, after detection of the test patches on the first or the second surface, when the detection result is fed back to change the image forming condition and the changing or correction effect needs to be confirmed by forming test patches again, another sheet needs to be conveyed again, which does not contribute to the reduction of the recording sheet.
JP-2005-215340-A discloses a configuration in which each of the first surface and the second surface is provided with an individual reading means. The same also discloses a technique in which an image forming condition for the first surface is obtained from the image formed on the first surface and that for the second surface is obtained from the image formed on the second surface, respectively, so that each image forming condition is made different. However, the use of two reading means increases both the size and the cost of the apparatus. Moreover, even in this case, another sheet needs to be supplied to confirm the correction result after test patches have been formed again.